Does Codecov support multiple languages in a single repository?

Is it safe to use the Codecov Bash uploader?

Yes. You trust install packages via pip, gem, npm and brew.
Running Codecov Bash is essentially the same concept. It all comes down to trusting the source of the package.
Contact support if you have any questions or concerns.

How does Codecov combine matrix builds and multiple CI providers?

Effortlessly. Codecov merges builds into a single report while maintaining the original source of the coverage data.
Send as many uploads from different CI providers and languages to Codecov. Plug-and-Play!
See a fine example here pyca/cryptography.

Does Codecov replace my CI provider?

No, Codecov does not run your test suite. That's the job of a CI provider.
Codecov does static analysis on your repository once your tests are complete and
coverage reports are uploaded.